1. Field of the Invention
The present invention relates to a device and method for driving a liquid crystal display, and, more particularly, to a method for driving a liquid crystal display having display cells each of which consists of a switching element connected to a data line and a gate line, a pair of electrodes oppositely disposed with a predetermined spacing, and a liquid crystal disposed between the pair of electrodes, and a driver for a liquid crystal display to which the driving method can be applied.
2. Related Art
A liquid crystal display (LCD) is conventionally known as a display for displaying characters or images such as graphics on an information processing device such as a personal computer. An active matrix driven LCD which is composed by arranging switching elements such as thin film transistors (TFT) in a matrix is particularly promising as a display replacing a CRT because it can surely control intensity of pixels and is also suitable for display of dynamic images with fast movement and color images. In a TFT LCD, a number of display cells are arranged in a matrix. Each display cell comprises a pair of TFT and an electrode provided on one of a pair of opposed substrates, a transparent common electrode formed over the entire surface of another substrate (transparent substrate), and liquid crystal sealed between the pair of substrates. The former substrate is provided with a number of gate lines for turning on the TFTs by row, and a number of data lines for applying voltage on the liquid crystal through the TFTs turned on.
A driving device for TFT LCD applies a voltage to the gate lines for a predetermined duration as shown in FIG. 10(A) to sequentially turn on the TFT of each display cell, and applies on each data line a voltage with intensity corresponding to a gradation value to be displayed on a display cell for which the TFT is turned on (data voltage) (see FIG. 10(B)). When the TFT is turned on and a data voltage is applied to the data line, the liquid crystal of each display cell varies its light transmittance according to the intensity of the voltage, and accumulates charges between the pair of electrodes. After the TFT is turned off, the accumulated charges maintain the state where the light transmittance is varied (see FIG. 10(C)). This causes an image to be displayed on the LCD. In addition, the life of liquid crystal is shortened if voltage with the same polarity continues to be applied. The life of liquid crystal is extended by driving the liquid crystal in a manner so as to periodically reverse the polarity of data voltage. For example, every one line or one frame the data voltage applied to each display cell is reversed utilizing the fact that the light is transmittance of liquid crystals becomes equal even if the polarity differs as long as the absolute value of the applied voltage is the same.
As a driver (a part of the driving device) for driving the data line of LCD, it is common to use a driver which has driving circuits for applying data voltage to the data lines in the same number as the data lines. However, when a very large number of data lines (for example, 1,000 lines) are provided to attain an LCD with a large surface area and high definition, which is an important challenge in the development of LCD, there arises a problem that the driver becomes very expensive because a very large number of driving circuits are necessary (for example, four driver ICs having 250 output terminals (250 driving circuits) are necessary to be provided to drive 1,000 data lines). In addition, when the pitch of LCD display cells is reduced to attain high definition of the LCD, the pitch of data lines is reduced as well. For example, if it is reduced to about 15 microns, it physically becomes very difficult to mount the driver ICs.
As a technique to overcome the above problems, it has been proposed to perform so-called multiplex driving wherein a driver having driving circuits of 1/n (n being an integer) in the number of data lines is provided, and a multiplexer is provided for selectively connecting the driver to any one of n sets of data line groups so that the driver and multiplexer sequentially drives n sets of data line groups. In such multiplex driving, because the number of driving circuits can be less than that of data lines, it is possible to keep the cost of the driver low, and mounting of the driver ICs can be relatively easy even if the pitch of data lines is small. However, in such multiplex driving, if the display cycle for an image (cycles of vertical and horizontal synchronizing signals) is constant, the time interval when the data voltage is applied to each of data line group is shortened as the ratio n of the number of data lines to the number of driving circuits (called the "multiplex ratio n") becomes larger. For example, as shown in FIG. 10(D), when the data lines are divided into four data line groups of A-D, and the data voltage is sequentially applied to display cells connected to each data line group, the duration to apply the data voltage to each display cell becomes 1/4 of the case where multiplex driving is not performed. This makes, as shown in FIG. 10(E), insufficient the duration to apply the data voltage (insufficient duration of application being represented by t.sub.1), and the voltage between the electrodes fails to reach the predetermined value corresponding to the applied data voltage (insufficient voltage between the electrodes being represented by V.sub.1) so that gradation provided by the light transmittance of liquid crystal of each display cell does not match the gradation to be displayed on each display cell, and thus the quality of the displayed image is deteriorated.
Therefore, in the conventional multiplex driving, it is common to provide a multiplex ratio of about 2 (the number of driving circuits being 1/2 of the number of data lines). This is because it is difficult to increase the multiplex ratio n in view of the reason described above.
The present invention is made in view of the above facts, and is intended to obtain a device and method for driving a liquid crystal display which can display an image at high quality even if the time to apply the data voltage is short.